Studies are conducted to obtain detailed understanding of the ultrasonic propagation properties of biological systems, with immediate emphasis on ultransonic absorption, attenuation, and velocity dispersion in mammalian tissues and organs and in solutions and suspensions of important biological macromolecules. Specific objectives are: (1) measurement of absorption and velocity of ultrasonic waves in tissues and in solutions of biopolymers, (2) identification of the physical parameters important for describing the observed behavior, (3) quantitative interpretation of the phenomena in terms of relaxation and/or other processes, and (4) continued development of appropriate instrumentation for obtaining the necessary experimental data. The attainment of these objectives contributes to an understanding of the physical mechanisms of interaction of sound and biological systems, which must be elucidate before the full potential of the ultrasonic techniques will be realized for medical diagnostic and therapeutic applications and for biochemical and biophysical investigation of fast reactions. The results of the investigations also contribute important associated with toxic effects and dosimetry of clinical ultrasound. Recent findings include the role of water and structural proteins in determining the propagation properties, in vivo measurements of absorption over wide ranges of temperature and frequency, propagation properties of lung, the influence of autolysis and fixation on attenuation, scattering and velocity, the influence of concentration on absorption, and the development of a small volume resonant instrument for determining absorption and velocity.